This invention relates to improvements in a magnetic field generating device for an electromagnetic flowmeter of residual magnetization type.
U.S. Pat. No. 4,409,846 discloses an electromagnetic flowmeter of residual magnetization type in which a semi-hard magnetic material which has a high permeability, which is easily magnetized and which has a coersive force of a relatively large value is used in its magnetic circuit.
Another electromagnetic flowmeter of residual magnetization type is also known in which its magnetic circuit includes magnetic materials providing a coersive part and a soft magnetic part as shown in FIG. 1.
The known electromagnetic flowmeter shown in FIG. 1 includes a pair of yokes 2 and 3 having their end faces disposed opposite to a fluid conduit 1 made of a non-magnetic material, and a core 4 disposed in magnetically series relation with the yokes 2 and 3. The core 4 is made of a magnetically semi-hard material so as to act as the coersive part described above, and the cores 2 and 3 are made of a magnetically soft material so as to act as the soft magnetic part described above. In the electromagnetic flowmeter shown in FIG. 1, its magnetic circuit is composed by the core 4, the yoke pair 2 and 3 and a magnetic gap including the fluid conduit 1, and a coil 5 is wound around the core 4. In order to magnetize the core 4 made of the semi-hard magnetic material, a pulse current having a short pulse width is periodically supplied to the coil 5 for a short period of time. The direction of this pulse current is alternately reversed. When the pulse current is supplied to the coil 5 in one direction, the core 4 is magnetized in a direction corresponding to the direction of the pulse current, and, when the pulse current disappears, a magnetic flux is applied across the fluid conduit 1 by the residual magnetization of the core 4. An electromotive force is generated according to the strength of the magnetic flux and the velocity of a fluid flowing through the fluid conduit 1 in a direction orthogonal with respect to the drawing sheet in FIG. 1. The electromagnetic force is derived from a pair of electrodes 6 and 7 and is sampled in an external electronic circuit (not shown) to measure the flow rate of the fluid. Since the direction of the pulse current supplied to the coil 5 is alternately reversed, the direction of the magnetic flux produced by the residual magnetization is also alternately reversed, and the direction of the electromotive force derived from the electrodes 7 and 8 is also alternately reversed.
In order to decrease the energy supplied to the coil 5 in the electromagnetic flowmeter of residual magnetization type shown in FIG. 1, that is, in order to decrease the power consumption, it is necessary to effectively utilize the residual magnetization of the core 4 of the semi-hard magnetic material disposed in the magnetic circuit. This is attained by determining the operating point of the magnetic circuit so that the operating point is set at a position where the magnetic energy product (B.times.H) in the B-H curve of the core 4 is maximum. In order that the operating point of the magnetic circuit in the electromagnetic flowmeter of residual magnetization type shown in FIG. 1 can be set at the point where the magnetic energy product (B.times.H) is maximum, the circuit constants of the magnetic circuit should be changed so that the operating point can be located at the desired position. This is attained by one of the following three methods:
(a) The magnetic gap between the yokes 2 and 3 is changed thereby changing the magnetic reluctance. PA1 (b) The sectional area of the core 4 is changed. PA1 (c) The axial length of the core 4 is changed.
However, the method described in (a) is impractical. This is because the diameter of the fluid conduit 1 cannot be decreased since the flow rate to be measured is fixed. Also, the increase in the magnetic gap undesirably results in an increased energy loss. Furthermore, it becomes undesirably necessary to change the dimension of the yokes 2 and 3. According to the method described in (b), an additional cost is required for changing the sectional area of the core 4. In addition, changing the sectional area of the core 4 requires changing the size of the coil 5 to meet the changed diameter of the core 4. Thus, the method described in (b) is costly and troublesome. According to the method described in (c), it is necessary to change the dimensions of the L-shaped yokes 2 and 3 to meet the changed length of the core 4, and it is also necessary to change the axial length lm of the coil 5 to meet the changed axial length of the core 4. Thus, the method described in (c) is costly and troublesome.